The present invention relates to a method for ciphering and deciphering data transmissions between a transmitter and at least one receiver of the type in which clear data are combined with key data, or streams, at the transmitting end, and at the receiving end the clear data are recovered by linking the ciphered transmission with identical key data and in which the same basic key is available in stored form at the transmitting end and at the receiving end.
The present invention serves not only to secure data against unauthorized access during the transmission between remote transmitters and receivers but also, for example, to secure data against unauthorized access during processing within a system where such unauthorized access to the data is to be prevented.
German Auslegeschrift [Published Application] No. 12 37 366 describes such a process in which the starting state of a code text generator is set in dependence on the result of mixing of a first state information sequence with a second state information sequence, the first state information sequence being generated at the transmitting end, preferably by means of a random, or pseudorandom, sequence generator, and being transmitted in the clear to the receiving end.
This process has the drawback that the rule for forming the binary signal sequence of the code text is fixed by the hard wiring of the code text generator and a change in the starting state merely permits the selection of another portion of the total code text sequence as the new code text. Moreover, a new additional code must be generated for every setting by means of a pseudorandom generator.
German Offenlegungsschrift [Laid-Open Application] No. 24 57 027 and counterpart U.S. Pat. No. 4,032,763 disclose a method for setting a code text generator in which a basic code word is determinative for the starting state of the code text generator and an additional code word in conjunction with the basic code word establishes the formation rules for the pseudorandom binary signal sequence generation of the code text.
This process thus does not operate according to a rigid formation rule for the binary signal sequence of the code text and is therefore significantly more secure against unauthorized decoding, but it would be much too expensive to use, if with the same security, it were to be used for transmission to a plurality of receivers which begin receiving at different times, i.e. if within short periods of time new additional codes would have to be repeatedly generated, transmitted and set.